Optical Transport Network (OTN) is defined by various standards to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals. Unlike OTN's predecessor Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH), OTN was designed for support of optical networking using wavelength division multiplexing (WDM). OTN is defined in, for example, ITU-T Recommendation G.709 (December 2009) “Interfaces for the Optical Transport Network (OTN)”, ITU-T Recommendation G.798 (October 2010) “Characteristics of optical transport network hierarchy equipment functional blocks”, and ITU-T Recommendation G.8251 (September 2010) “The control of jitter and wander within the optical transport network (OTN)” the contents of each is incorporated by reference herein. Also unlike SONET/SDH, OTN does not distribute a network clock network wide. OTN started off as a point-to-point digital wrapper mainly for OAM&P and FEC purposes. OTN timing can come from two different un-synchronized clock sources, namely a shelf clock or from a client (i.e., line timing). Client clocking is used when the client consumes the whole payload and configuration allows OTN to be bit synchronous to client. Shelf timing is used when an OTN line is created locally to a node and contains multiplexed structure.
Clock adjustments in an OTN context can be problematic. For example, clock adjustments can include switching between the shelf timing and line timing and vice versa. These adjustments can occur during mode changes on an OTN line, i.e. changing from low order (LO) Optical channel Data Unit level k (ODUk) to high order ODUk and vice versa. That is, mode changes can occur when switching between 1:1 mapped ODUk containers and multiplexed ODUk containers. Specifically, lines affected by such clock adjustments experience transient incoming alignment errors (IAE), out-of-frame (OOF), etc. Problematically, these transients travel across the network (end-to-end) as the client signal is switched into the line (or vice versa). As each line changes clock sources from shelf to line, each down-stream line experiences a small, but alarmed outage. For example, a last link of an n-hop connection could experience n IAEs. Backward paths receive indications of the forward path framing difficulties as consequent actions (BIAE) as well. Synchronous (SONET/SDH) networks do not suffer from this condition. If nothing is done, then users can experience spurious alarms when ODUk services are added. In addition to these transients creating IAE events, data path errors can also be experienced leading to link failures that lead to restoration events and/or failures in in-band messaging (i.e. GCC overhead corruption).